I. Field of the Invention
This invention relates to radius dressers which are used to shape or "dress" precision grinding wheels of the type used in the manufacture of ball bearing assemblies and other precision parts. More particularly, this invention relates to the precision bearings required for such grinding wheel dressers, and specifically to molded bearings for use in such applications.
II. Description of Related Art
Modern grinding techniques are capable of grinding parts to tolerances of 20 millionths of an inch or better. However, to achieve this accuracy, the grinding wheel must be dressed to the same or better accuracy. Accordingly, when producing a curved surface on the final part, as is required on the races of ball bearings, the grinding wheel must be shaped by a radius dresser capable of moving the dressing tool along a curved path which is accurate to the desired tolerances.
Most modern radius dressers designed for such applications mount the dressing tool on a spindle held in ball bearings. As the spindle rotates, the dressing tool moves in a precision arc which forms the curved surface on the spinning grinding wheel. Any imperfection in the spindle bearings is directly transferred to the grinding wheel and from there to each ground part. As a result, although they are expensive, Class 7 angular-contact ball bearings are normally used to hold the spindle because they have the necessary precision for this application.
Unfortunately, ball bearings are subject to damage by impact which results in poor quality of the final part and expensive downtime as the bearings are replaced. This susceptibility to damage is the result of the design of ball bearings (and some other bearings) in which the high accuracy is achieved by limiting the contact area between the rotating components of the bearing to a relatively small high precision area along the races and the surface of the balls.
In ball bearings, the balls are essentially in point contact with the inner and outer races. All of the load carried by the bearing passes through these point contacts, which causes an extremely high load per unit area. Moreover, the load is usually carried by only one or two of the balls, and thus passes through very few point contact areas.
Consequently, if the grinding wheel dresser suffers an impact or "dresser wreck", the load at the point contacts of the ball bearing may exceed the load limit supportable by the bearing race material which permanently deforms the bearing race or the balls.
After such an impact the bearing race and/or the balls no longer have the required precision. As they rotate, they repeatedly contact the deformed area producing a corresponding motion by the dressing tool which is reproduced on the surface of the grinding wheel and ultimately on the surface of the part to be ground. This problem is most significant at the bearing closest to the dressing tool, as the bearing at this location suffers the highest loading and has the greatest effect on the precision of the dressing tool motion.
Nonetheless, in prior art designs, this disadvantage has been outweighed by the difficulty of producing a bearing having the necessary precision over a larger contact area that would spread the impact load and avoid deforming the bearing. Bearing designs other than ball bearings have been proposed, but they have still relied upon a small contact area between the rotating bearing parts to achieve the necessary precision. Simply stated, the smaller the bearing contact area, the easier it is to maintain the necessary accuracy, but the more susceptible to damage is the bearing.
Accordingly, a principal object of the present invention is to provide a bearing suitable for use in a precision radius dresser which is resistant to damage from impact.